A step in the biosynthesis of gibberellins that is controlled by the mutation in the semi-dwarf rice cultivar Tan-ginbozu

Genetic analysis and a comparison of endogenous levels of gibberellins between the semi-dwarf rice cultivar Tan-ginbozu and the corresponding normal cultivar Ginbozu have confirmed that Tan-ginbozu is a gibberellin-deficient mutant and that the semi-dwarfism of Tan-ginbozu is controlled by a single recessive gene, A step in the biosynthesis of gibberellins that is blocked by the mutation in Tan-ginbozu had been considered to be the synthesis of ent-kaurene or an earlier step. However, the rate of production of ent-kaurene by Tan-ginbozu was almost the same as that by Ginbozu. By contrast, accumulation of only a small amount of ent-kaurene was detected in Tan-ginbozu, and the amount that accumulated was similar to that in Ginbozu that had been treated with 6.9 x 10(-8) M uniconazole-P (an effective inhibitor of three oxidative steps in the pathway from ent-kaurene to ent-kaurenoic acid via ent-kaurenol and ent-kaurenal), The height of the treated Ginbozu plants was reduced to the same as that of Tan-ginbozu plants, Resembling Tan-ginbozu plants, Ginbozu plants that had been treated with uniconazole-P responded well to ent-kaurenoic acid and slightly to ent-kaurene and ent-kaurenol, Since the growth-promoting activity of ent-kaurenal in Tan-ginbozu was similar to that of ent-kaurene, our results suggest that the mutation in Tan-ginbozu blocks the three oxidative steps whereby ent-kaurene is converted to ent-kaurenoic acid.